California health officials have confirmed an additional 21 human West Nile virus (WNV) cases during the past week, bring the state’s total to 57 this year from 13 counties to date. The new cases were reported from the following counties: Butte (14), Glenn (2), Lake (1), Mendocino (1), Nevada (1), and Tehama (2).

Butte County, located in the California Central Valley, north of the state capital of Sacramento, has seen more than half the cases thus far with 30.

To date, California has reported two WNV-related fatalities– from San Bernardino and Nevada counties.

Also, 14 asymptomatic WNV positive blood donors have been reported this year from eight counties.

As of August 18, 210 cases of WNV human disease have been reported from 29 states nationally. Of these, 126 (60%) were classified as neuroinvasive disease and five fatalities have been reported.

West Nile virus is transmitted to humans and animals by the bite of an infected mosquito. For most people, the risk of serious illness is low. However, some individuals – less than one percent – can develop a serious neurologic illness, such as encephalitis or meningitis.

People 50 years of age or older and people with diabetes and/or high blood pressure have the greatest risk of developing serious complications.

Robert Herriman is a microbiologist and the Editor-in-Chief of Outbreak News Today